(C₅N₂H₇)IO₂F₂ and (C₃N₆H₈)(IO₂F₂)₂: two new organic-inorganic hybrid fluoroiodate birefringent crystals

Birefringent crystals are anisotropic materials commonly used in modern optical devices to obtain polarized light. The traditional commercial birefringent crystals, such as MgF₂ and CaCO₃, have low birefringence and are not easy to grow. Herein, two organic-inorganic hybrid birefringent crystals composed of π-conjugated organic groups and lone pair-containing (IO₂F₂)^- anions, namely, (C₅N₂H₇)IO₂F₂ and (C₃N₆H₈)(IO₂F₂)₂, were successfully grown by a simple evaporation method. In their structures, the π-conjugated organic cations and (IO₂F₂)^- anions are interconnected through hydrogen bonding and π-π stacking interactions into 3D supramolecular networks. Both of them exhibit large birefringence values of 0.30 @ 550 nm and 0.23 @ 550 nm, which are superior to the values of most of the conventional birefringent materials. Moreover, UV-vis-NIR diffuse reflectance spectra demonstrate that they have wide band gaps (4.06 eV and 4.10 eV) because of the presence of the F element with strong electronegativity. Theoretical analyses suggest that the parallel arrangement of the π conjugated cations resulted in large optical anisotropy. This work provides new promising candidates for UV birefringent materials.